Serial Gateway freezes on Raspberry Pi's USB Port

I have an Arduino Pro Mini (3.3V) connected to an NRF24l01 acting as my Serial Gateway. As a first sensor I use the same configuration with a DHT22 attached to it. And it all works fine, as long as the Gateway is connected to my MacBooks USB port (via an FTDI board btw.)
However, I connected the gateway to my Raspberry to test it there and the gateway hangs after:

Hi Andre, I'm about to build a serial GW for my controller (Rasp Pi 3 running the latest build of Domoticz). I'm a little confused how the GW will practically interface with my controller however. Do i just plug the GW into one of my RPi's USB ports?? Is it as simple as that? Thanks for answering my "newbie" questions!!

The serial Port on the current Raspberry Pi 3 is on Pins 8/10. That is the big row of pins on the board. With the current Raspbian you have to first add enable_uart=1 to the cmdline in /boot/config.txt.
After a reboot /dev/ttyS0 becomes available. You need to disable getty on /dev/ttyS0 using systemctl mask serial-getty@ttyAMA0.service. Otherwise, it will interfere with communication between your gateway and the Pi.

When set up properly, the serial interface can be checked for data using terminal software such as picocom or minicom (seriously, use picocom ;)).